Path difference in Fabry-Perot etalon

Hi
I'm trying to understand how the Fabry-Perot etalon works for an experiment regarding the Zeeman effect.
Looking at the image could anyone tell me why the path difference in the etalon is [tex]\Delta l = 2t \cos{\theta}[/tex]? In my head, the path difference is [tex]\Delta l = 2t/\cos{\theta}[/tex], in other words, just the length of two more reflections.